1. Field of the Invention
The present invention relates to electronic and hydraulic control system of a 4-speed automatic transmission for an automotive vehicle and a method for controlling hydraulic pressure.
2. Description of Related Art
In general, an automatic transmission provides a necessary gear ratio to operate the vehicle under a wide range of speeds and loads. It does this with a minimum amount of effort on the part of the driver. That is, automatic upshifts and downshifts are a convenience for the driver because a foot-operated clutch is not required to shift gears and because the vehicle can be brought to a stop without the use of a clutch and without shifting the transmission into neutral.
A conventional automatic transmission for a vehicle comprises a torque converter having an impeller, a turbine and a stator, a gear train connected to the torque converter to provide various forward speed ranges and reverse, a plurality of friction members such as disc clutches, one-way clutches which control gear action, and a hydraulic control system for controlling the operation of the friction members. The hydraulic control system includes a plurality of solenoid valves which are controlled by a transmission control unit (TCU).
However, since the hydraulic control system adapted to the conventional 4-speed automatic transmission can not provide a skip shifting, e.g. from the fourth speed to the second speed, responsiveness with respect to a gear shifting is retarded.
Additionally, the system is designed such that only two predetermined modes of pressure, that is, pressure of a drive "D" range and pressure of a reverse "R" range, are fed to the pressure line, thereby causing drive loss of the hydraulic pump and deteriorating the fuel consumption ratio.
Additionally, the torque converter is providing with a damper clutch for directly transmitting power of the engine to the gear train to thereby improve transmission efficiency.
The damper clutch is controlled by a transmission control unit (TCU) receiving a temperature signal of operating pressure, an opening signal of the throttle valve of engine, and a switch signal of the damper clutch brake.
The damper clutch is applied at second, third and fourth speeds of the drive "D" range. The line hydraulic pressure is varied when the damper clutch is applied, thereby increasing transmission efficiency. However, if a minimum line hydraulic pressure cannot be maintained, the friction member gets damaged.
Further, the transmission control unit is designed to output a signal for each speed ratio in accordance with a shift pattern. However, after outputting the signal, the TCU cannot confirm that the friction member is correctly applied. Accordingly, it is impossible that the shift operation is correctly performed.
That is, when hydraulic pressure applied to each friction member is insufficient, shift quality is deteriorated, or friction wear is significant. In the conventional art, since no means for detecting an amount of hydraulic pressure applied to the friction member is provided, the hydraulic pressure cannot be compensated.